This is a study of the physiology and pharmacology of spike frequency adaptation in the neurons of Aplysia californica. Certain neurons of Aplysia exhibit spike frequency adaptation in two phases. The rapid phase is a steep decline of neuronal firing frequency following the onset of a depolarizing current and is complete withing 10 seconds. It appears to be determined by the inactivation of a slow inward current. This rapid phase of adaptation is enhanced by the anticonvulsant diphenlyhydantoin. The second phase of spike frequency adaptation is much longer, resulting in a continuing decline of spike frequency lasting minutes. This slow phase is produced by a slow potassium current, and is dramatically enhanced by barbiturates. This project is directed to understanding the electrophysiological mechanisms underlying the adaptation process and the ways in which these anticonvulsant drugs modify these mechanisms.